Cobalt(III)-Catalyzed Directed C-H Coupling with Diazo Compounds: Straightforward Access towards Extended π-Systems

Article abstract of DOI:10.1002/anie.201411994

The first highly efficient and scalable cobalt-catalyzed directed C-H functionalization with carbene precursors is presented. This methodology provides a modular route towards a new class of conjugated polycyclic hydrocarbons with tunable emission wavelengths both in solution and in the solid state.

Full text of DOI:10.1002/anie.201411994

Angewandte
Chemie
DOI: 10.1002/anie.201411994
Conjugated Polycycles
À
Cobalt(III)-Catalyzed Directed C H Coupling with Diazo
Compounds: Straightforward Access towards Extended p-Systems**
Dongbing Zhao,* Ju Hyun Kim, Linda Stegemann, Cristian A. Strassert, and Frank Glorius*
Dedicated to Professor Gꢀnther Wilke on the occasion of his 90th birthday
Abstract: The first highly efficient and scalable cobalt-
catalyzed directed CÀH functionalization with carbene pre-
transition-metal-based catalysts may demonstrate unique
reactivity not observed with noble-metal-based catalytic
systems and expand the scope of directed CÀH functionaliza-
cursors is presented. This methodology provides a modular
route towards a new class of conjugated polycyclic hydro-
carbons with tunable emission wavelengths both in solution
and in the solid state.
tion.
Carbenoid insertion into CÀH bonds is a well-established
[
7]
method to functionalize inert CÀH bonds.
However,
directed CÀH bond functionalization involving CÀH metal-
C
onjugated polycyclic hydrocarbons have received wide-
ation, metal-carbene formation, and migratory insertion still
[
8]
spread attention in recent years because of their interesting
and varied electronic, optoelectronic, and magnetic proper-
has limited precedent. In 2012 Yu demonstrated the first
example with an ortho-alkylation of directing-group-contain-
[
1]
III
ties. The design and synthesis of novel ring-fused conjugated
polycyclic hydrocarbons to access systems with attractive
electronic structures and solid-state packing properties is
strongly desired. Direct p-extension of simple aromatics using
CÀH activation strategies is one of the most rapid, efficient,
ing arene CÀH bonds with diazomalonates using a Rh
[
9]
catalyst (Scheme 1a). More recently, Rovis, Li, Glorius,
Wang, and Cui have demonstrated the successful exploration
of diazo compounds in Rh -catalyzed directed CÀH func-
III
and convenient ways to obtain diverse fused p-conjugated
[2]
polycyclic hydrocarbons.
In recent years, noble-metal-catalyzed directed CÀH
functionalization has emerged as a versatile approach in
[
3]
organic synthesis. However, because of the relatively high
cost of these noble metals, the development of inexpensive
earth-abundant first-row transition metals as alternative
catalysts is attracting increasing attention in directed CÀH
[4]
II
functionalization. For example, Co -catalyzed CÀH activa-
tion pioneered by Nakamura, Ackermann, and Yoshikai has
recently been applied in various transformations traditionally
[
5,6]
catalyzed by Rh, Ru, and Pd.
Furthermore, these first-row
[
+]
[+]
[
*] Dr. D. Zhao, Dr. J. H. Kim, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: dongbing.zhao@uni-muenster.de
glorius@uni-muenster.de
Scheme 1. Directed CÀH bond functionalization with carbene precur-
sors.
[
10]
tionalizations (Scheme 1a). However, although Co is more
earth-abundant and less expensive than Rh, cobalt-catalyzed
directed CÀH functionalization with carbene precursors has
never been reported. Until now only cobalt-catalyzed CÀH
L. Stegemann, Dr. C. A. Strassert
Physikalisches Institut and Center for Nanotechnology (CeNTech)
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster (Germany)
+
[
] These authors contributed equally to this work.
[
**] This work was supported by the Alexander von Humboldt
functionalization of 1,3-azoles with carbene precursors has
[11]
Foundation (D.Z.), the Korea Research Foundation (NRF-
been reported (Scheme 1b).
2014R1A6A3A03057520, J.H.K.), the European Research Council
Very recently, Kanai, Ackermann, Ellman, and Glorius
under the European Community’s Seventh Framework Program
III
demonstrated that the Cp*Co catalyst could promote some
(
(
FP7 2007-2013)/ERC Grant Agreement 25936 and by the DFG
Leibniz Award). We thank Dr. Constantin G. Daniliuc for X-ray
III
reactions already established with Cp*Rh catalysts to access
[
12]
the identical products. More interestingly, Kanai further
crystallographic analysis and Prof. Shigehiro Yamaguchi (Nagoya
University), Dr. Chang Guo, Dr. Matthew Hopkinson, and Tobias
Gensch (all WWU Mꢁnster) for helpful discussions.
III
proved that a Cp*Co catalyst could also trigger reactivity
III
differing from that of the related Cp*Rh catalysts because of
the large difference in electronegativity between Co and
Rh, demonstrating the potential utility of Cp*Co catal-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411994.
[
13]
III
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ysis. Based on these results and our previous work, we
speculated that a cobalt(III) catalyst might also promote
directed CÀH functionalization with carbene precursors to
We next examined the scope of N-heteroarylarenes in the
CÀH bond annulation (Scheme 3). First, various 2-arylpyr-
idines proceeded smoothly to afford the desired products in
satisfactory yields irrespective of the substrate electronics
(Scheme 3, 4a–t). Secondly, our method is suitable for not
give products distinctly different from those obtained with the
III
related Rh catalytic systems. Herein, we disclose the first
cobalt-catalyzed directed CÀH functionalization with carbene
precursors, which is highly efficient and scalable and provides
a modular route for the assembly of an interesting class of
structurally diverse conjugated polycyclic hydrocarbons,
which have tunable emission wavelengths both in solution
and in the solid state (Scheme 1c).
In preliminary experiments, 2-phenylpyridine 1a was
treated with [Cp*Co(CO)I ] (5 mol%), AgSbF (10 mol%),
2
6
KOAc (20 mol%), and diazo ester 2a (1.5 equiv) in methanol
1 mL) at 1008C for 24 h. Unfortunately no coupled product
(
was observed under these conditions and a survey of different
solvents was carried out (Table S1). When we used 2,2,2-
trifluoroethanol (TFE) as the solvent, product 3a was
obtained in 33% yield. We also attempted reactions with
III
Cp*Rh species in the presence and absence of KOAc, but in
all cases only the alkylated products were obtained as the
[9]
main products (entries 5–8, Table S1).
Increasing the
amount of [Cp*Co(CO)I ] and the reaction temperature to
2
1
208C improved the yield of 3a to 72% (see the Supporting
Information).
Subsequently, the scope of the diazo esters was inves-
tigated using 2-phenylpyridine 1a as the substrate. As shown
in Scheme 2, a series of 6H-pyrido[2,1-a]isoquinolin-6-ones
bearing diverse substituents at the C7 position were synthe-
sized in good to excellent yields. It is important to stress that
these reactions using diazomalonates as substrates only
Scheme 3. Scope of arenes and heteroarenes in the tandem CÀH bond
activation/annulations. For reaction conditions, see the Supporting
Information.
worked smoothly with [Cp*Co(CO)I ] (2.5 mol%), AgSbF6
2
(
(
5 mol%), and KOAc (10 mol%) as co-catalysts in TFE
Scheme 2, 3g–i). For the coupling of 2-phenylpyridine 1a and
only diverse monosubstituted 2-arylpyridines, but also for
disubstituted 2-arylpyridines (Scheme 3, 4n–p). Additionally,
the arene is not limited to a benzene platform; heteroarenes
also undergo the same reaction with excellent levels of
positional selectivity (Scheme 3, 4q). Moreover, a variation of
the substitution pattern on the pyridine moiety proved to be
tolerated without significantly affecting the catalytic efficacy
dimethyl diazomalonate 2g, the catalyst loading could be
further decreased to 1 mol% without a significant decrease in
yield (81%). The structure of 3 f was confirmed by X-ray
crystal analysis. In addition, the reaction can be conducted
on a gram scale (1 g of 3g, 82% yield).
[14]
(Scheme 3, 4r–t). Benzo[h]quinoline also showed high reac-
tivity in this transformation (Scheme 3, 4u). We were pleased
to observe that pyrimidine could also be utilized as a directing
group to obtain the corresponding cyclic product 4v
(Scheme 3). In contrast, when 1-(pyrimidin-2-yl)-1H-indoles
were employed as substrates, the alkylated products 5 were
obtained in high yields (see Scheme S1).
Gratifyingly, the CÀH coupling of diazo compounds
occurred not only with N-heteroarylarenes but also with
olefins via alkenyl CÀH activation. 2-(Propenyl)pyridine 1w
showed good reactivity in this reaction selectively delivering
the 4H-quinolizin-4-one core, which is emerging as a key
[15]
pharmacophores in a range of biological targets [Eq. (1)].
To probe the reaction mechanism, a series of experiments
were carried out (see the Supporting Information). Competi-
tion experiments were used to determine the electronic
preference of the reaction. When p-Cl-phenylpyridine 1e was
run in competition with 1a, the reaction favored 3g in a 3.15:1
ratio, implying that an electrophilic CÀH activation may be
Scheme 2. Scope of diazo esters. For reaction conditions, see the
Supporting Information.
2
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Chemie
operating. The use of a deuterated co-solvent proved that the
CÀH activation was reversible. The value of k /k = 1.0 from
H
D
two parallel reactions indicated that the CÀH metalation step
is not rate-determining. A competition experiment was also
used to determine the preference for electronically different
diazo compounds. The result suggests that migratory insertion
favors a more electron-deficient diazo substrate (see the
Supporting Information).
Based on previous reports and our preliminarily mecha-
[9,10,12,13]
nistic experiments,
we propose a directed reversible
CÀH bond cleavage to form cobaltacycle II as the first step
Figure 1. Absorption (a) and emission (b) spectra in CH Cl at RT
(10 m). Observed fluorescence under UV excitation (365 nm) (c).
2
2
III
À5
after generation of the active Cp*Co catalyst I. Then further
reaction with the diazo compound might occur to form the
metal–carbene intermediate III by dediazonization
(
Scheme 4). Subsequently, cobalt-carbene migratory insertion
are depicted in Figure 1 and Figures S1–S4. The PL range
covers the whole visible region, offering a broad palette of
in intermediate III affords the cobaltacyclic intermediate IV,
colors both in CH Cl2 (lem
:
471–727 nm,
Table S2 and Figure 1c) and in the solid state
lem: 567–714 nm, Table S3). Most of these
compounds display high values
2
(
e
4
À1
À1
(
ꢀ 10 m cm ) for the lowest electronic tran-
sitions. Extension of the p-conjugated systems
leads to significant bathochromic shifts in
[16]
emission (4u and 4p, Table S2 and Figure 1).
In conclusion, we have successfully devel-
oped a cobalt(III)-catalyzed coupling of diazo
compounds with aromatic and heteroaromatic
compounds via CÀH activation. The observed
difference in the catalytic reactivity and activity
III
III
between the Cp*Co and Cp*Rh complexes
points to the dual role of the Co species as
a transition metal and a Lewis acid catalyst.
Furthermore, this facile synthetic approach
resulted in the discovery of a new family of
Scheme 4. Proposed reaction mechanism.
extended p-systems. Most of these compounds
which undergoes proto-demetalation and Lewis acid pro-
moted nucleophilic addition to form intermediate VI. Finally,
aromatization gives the desired product and regenerates the
active species I. The dual role of cobalt as a transition metal
and a Lewis acid catalyst might be vital to the CÀH activation
have broad and intense absorption and emission bands, and
exhibit high extinction coefficients and large Stokes shifts.
Received: December 14, 2015
Published online: && &&, &&&&
step and sequential cyclization in this transformation.
Keywords: carbenes · CÀH activation · cobalt · heterocycles ·
All of the synthesized compounds are brightly fluorescent.
To further understand the photophysical properties of the
involved electronic excited states, we characterized a series of
representative derivatives in solution at RT, in frozen glassy
matrices at 77 K, and in the solid state (powders). The
absorption and excitation maxima, the molar absorption
coefficients (e), the photoluminescence (PL) maxima and
quantum yields (F ), as well as the excited state lifetimes (t )
.
p-systems
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Supporting Information.
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Conjugated Polycycles
D. Zhao,* J. H. Kim, L. Stegemann,
C. A. Strassert,
F. Glorius*
&&&& — &&&&
Cobalt(III)-Catalyzed Directed CÀH
Coupling with Diazo Compounds:
Straightforward Access towards Extended
p-Systems
Co-catalyzed! The first highly efficient and
scalable cobalt-catalyzed directed CÀH
a modular route towards a new class of
conjugated polycyclic hydrocarbons with
tunable emission wavelengths.
functionalization with carbene precursors
is presented. This methodology provides
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!